This invention relates to floating offshore storage, drilling or production platforms and more particularly, platforms of the Spar type. More specifically, the invention relates to an improved interface connection assembly between the Spar hull and a caisson extending downwardly therefrom.
Floating platforms are typically employed in water depths of about 500 ft. (approximately 152 m) and greater, and are held in position over a seabed well site by mooring lines anchored to the sea floor, or by motorized thrusters located on the sides of the platform, or both. Although floating platforms are more complex to operate because of their movement in response to wind and water conditions, they are capable of operating in substantially greater water depths than are fixed platforms, and are also more mobile, and hence, easier to move to other well sites. There are several different types of known floating platforms, including so-called “drill ships,” tension-leg platforms (“TLPs”), “semi-submersibles,” and “Spar” platforms.
Spar-type platforms comprise an elongate, substantially cylindrical (or multi-cylindrical) buoyant hull that supports one or more decks above the surface of the water when floating in an upright operating position, in which an upper portion of the platform hull extends above the waterline and a lower portion is submerged below it. Because part of the platform hull is above the water, it is subject to forces of wind, waves, and water currents which cause a constant movement of the platform. Generally, a Spar-type floating platform is designed to be installed for a 25 to 30 year service life.
Despite their relative success, Spar-type platforms include some aspects that need improvement to accommodate various applications and/or a larger spectrum of environmental conditions. For example, in typical Spar designs and configurations, at least one tubular caisson extends downwardly from the bottom of the Spar hull. The caisson(s) may extend some distance below the Spar, or to the sea floor. Often the inside of the caisson will include a plurality of tubulars and/or control bundles. Because the caisson extends below the floating spar hull, there is typically relative, flexing motion at the interface connection area between the Spar hull and the caisson. Currently existing hull-to-caisson interface connections do not accommodate very high tensile loads, the bending/flexing relative motion between the spar hull and the caisson for the life of the installation, and the very demanding fatigue life. Since the wind, waves, and water currents are always present in various intensities, the hull/caisson interface connection must be designed to accommodate the very demanding fatigue life due to the high loads and constant relative motion.
It would therefore be desirable for an improved Spar hull-to-caisson interface connection that accommodates high tension loads between the spar hull and the caisson while still facilitating angular, flexing motion (rotation) while also simultaneously constraining lateral movement and vertical axial movement. The result would be highly-improved fatigue characteristics of the interface between the Spar hull and the caisson.